Your weight is 45 kg which is low.
Firstly you increase your wt.about 6-8 kg,
take high protein,fat,carbohydrate
take milk with high fat,dry fruits,fresh fruits and green veges.
After increasing wt.you start exercise like wt lifting,pull ups,which build up your biceps
and chest muscles(pectoral),leading to six packed body.
ok and bye.